a) Field of the invention:
The present invention relates to a vari-focal lens system which is to be used with cameras and so on.
b) Description of the prior art:
It has recently become common to use zoom lens systems as photographic lens systems for silver salt photographic cameras, video cameras and so on. These zoom lens systems have a common defect; they require a large of lens elements for correcting aberrations and have large external dimensions.
In order to correct this defect, aspherical lens elements are used. However, it is impossible to remarkably reduce the number of lens elements by the use of aspherical lens elements since aspherical surfaces are ineffective for correcting chromatic aberration though they can correct spherical aberration, coma, distortion, etc.
A vari-focal lens system consists of a plurality of lens units, in each of which chromatic aberration must be corrected. For this reason, each lens unit must comprise at least two lens elements, i.e., a positive lens element and a negative lens element even when aspherical surfaces are used in the lens unit.
There are used in these days graded refractive index lens elements (GRIN lens elements) which have refractive indices varying between portions thereof. The use of the GRIN lens elements makes it possible to correct spherical aberration, etc. like use of the aspherical lens elements, and at the same time to correct curvature of field and chromatic aberration. A lens system which has a fixed focal length and comprises GRIN lens elements is reported on pages 993 and later in Applied Optics, Vol. 21. This lens system favorably corrects aberrations with two radial GRIN lens elements having the shapes of concave lens elements, made of media having positive refractive powers and arranged concentrically with an aperture stop as shown in FIG. 1, and is composed of lens elements in a number smaller than half the number of the lens elements used in the conventional lens system.
Further, as an example of vari-focal lens systems using GRIN lens elements, there is known the lens system disclosed by U.S. Pat. No. 4,776,679. However, a remarkable reduction of the number of lens elements is not made in this vari-focal lens system.